The present invention relates to a method for protecting an SiO2 coating on boundary surfaces of a combustion chamber to which a combustion medium is fed, and to a device for combustion of a combustion medium, having a combustion chamber, in which a combustion operation takes place, and a burner for supplying the combustion medium, boundary surfaces of the combustion chamber having an SiO2 coating, in particular for carrying out the method according to the invention. It also relates to a combustion medium for use in a method of this type or a device of this type.
An SiO2 layer is formed on materials which are exposed to high temperatures, in particular on (Si-containing), non-oxidic and Si-containing, oxidic high-temperature materials. The layer protects the materials in operation. In practice, however, corrosion occurs when these materials are used in combustion chambers. The reason for this is that the composition of the flue gases that are formed prevents a protective layer of this type from building up or dissolves a protective layer that is already present. Particularly in the event of interaction with steam, SiO is formed, a compound which is volatile on account of its high vapor pressure, reduces the thickness of the SiO2 layer and therefore leads to a constant attack on the base material. In the case of oxidic materials, the material composite at the surface is broken down by the evaporation of the SiO2 protective layer. Therefore, despite their otherwise highly favorable high-temperature properties, these materials have only a limited service life.
It is accordingly an object of the invention to provide a method for protecting an SiO2 coating, and a combustion device with the protection that overcome the above-mentioned disadvantages of the prior art methods and devices of this general type.
With the foregoing and other objects in view there is provided, in accordance with the invention, a method for protecting an SiO2 coating on boundary surfaces of a combustion chamber being fed a combustion medium. The method includes adding an additive to the combustion chamber for increasing a SiO content of flue gases formed in a combustion operation.
According to the invention, in the method of the type described in the introduction, the object is achieved through the fact that an additive that increases the SiO content of the flue gases formed in a combustion operation is added to the combustion chamber. The device according to the invention provides an additional device for adding an additive that increases the SiO content of the flue gases formed in the combustion operation.
The SiO content of the flue gases is increased by the addition of the additive. The evaporation from the existing SiO2 coating is significantly reduced or completely prevented as a result of this increase.
The additive that is added is advantageously an organosilicon compound, an ester of silicic acid or a mixture thereof. The additives are inexpensive and are easy to handle.
In an advantageous configuration, the additive is added in an amount that corresponds to 1 ppm to 1% by weight, in particular approximately 10 ppm to 1% by weight, of the combustion medium that is fed to the combustion operation. The quantity that is actually added is dependent on the boundary conditions, such as for example the temperature and the pressure in the combustion chamber and any natural level of SiO, SiO2, an organosilicon compound or an ester of silicic acid that may be present in the combustion medium. Since the quantity of the additive that is added is very low, the combustion operation itself remains virtually unaffected.
In the case of combustion operations that take place under substantially steady-state conditions, the required quantity of the additive can be determined on a one-off basis, and this defined quantity can then always be added. There is no need for complex and detailed measurements or calculations.
On the other hand, according to an advantageous refinement, in the case of fluctuating combustion operations and/or by way of example, quantities of fuel that fluctuate over the course of time, the SiO content of the flue gases is measured, and the amount of additive which is added is regulated as a function of the measured value. This always allows the desired SiO content of the flue gases to be maintained even in the event of fluctuating boundary conditions.
As an alternative or in addition, it is possible to measure a parameter of the flue gases from which the vapor pressure of the SiO2 coating can be calculated. The quantity of additives that is added is once again regulated as a function of the measured value. Measuring the SiO content of the flue gases is relatively complicated. In contrast, the vapor pressure of the SiO2 coating can be calculated on the basis of the temperature and pressure in the combustion chamber. Both temperature and pressure are easy to measure. Furthermore, the temperature of the combustion operation is generally already being measured in order to maintain the predetermined operating state and to minimize the emission of pollutants. The additional outlay is therefore low.
In a first advantageous configuration, the additive is added to the combustion chamber separately from the combustion medium. In this procedure, any desired additive in liquid, gaseous or pulverized form can be used. The properties of the additive do not have to be matched to the combustion medium.
According to a second advantageous configuration, the additive is admixed with the combustion medium and added to the combustion chamber together with the combustion medium. In this case, only a single addition device is required for the combustion medium and the additive.
According to an advantageous refinement, the additive is soluble in the combustion medium. This facilitates the addition of the additive. There is no precipitation of the additive even if the addition of combustion medium stops briefly. The additive can in this case be added to the combustion medium as early as at the refinery, so that there is no need for any changes in the combustion chamber.
It is advantageous for the additive to be combustible. This prevents any deterioration to the efficiency as a result of the introduction of incombustible substances to the combustion chamber.
With the foregoing and other objects in view there is provided, in accordance with the invention, a device for combustion of a combustion medium. The device contains a combustion chamber for carrying out a combustion operation, the combustion chamber having boundary surfaces with an SiO2 coating. A burner is provided for supplying the combustion medium and is disposed in the combustion chamber. An adding device is provided for adding an additive for increasing a SiO content of flue gases formed in the combustion operation.
In an advantageous refinement of the device according to the invention, there is at least one sensor for measuring the SiO content and/or the temperature of the flue gases in the combustion chamber. The sensor can be used to measure the SiO content or to calculate the vapor pressure of the SiO2 coating. The quantity of additive that is added is regulated as a function of the measure value.
The burner of the device has either a nozzle for adding the additive separately from the combustion medium or a nozzle for supplying a mixture of additive and combustion medium. In the first configuration, the additive can be added independently of the combustion medium and therefore does not necessarily have to be matched to the latter. The second configuration advantageously uses an additive that is soluble in the combustion medium. The burner of the device may, for example, be configured as a burner of a gas turbine, and the combustion chamber can accordingly be configured as a gas-turbine combustion chamber.
It is advantageous to provide a pump for metering the quantity of additive that is added. The pump allows a targeted, in particular accurately metered feed of the quantity that is desired in each instance.
An additive that increases the SiO content of the flue gases formed in a combustion operation is admixed with the combustion medium according to the invention. The additive may be admixed as early as in the refinery or if appropriate at a later time. The procedure is recommended if large quantities of the combustion medium are being taken off and the additive can be held in the combustion medium for prolonged periods without suffering any deterioration. In this case, there is no need for any structural changes to the burner itself, which is a particularly cost-effective option.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a method for protecting an SiO2 coating, and a combustion device with the protection, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.